OFDM networks support the transmission of both broadcast traffic, intended for multiple subscriber stations (i.e., user devices), and unicast traffic, intended for a single subscriber station. Conventional OFDM networks time-multiplex broadcast and unicast traffic in the downlink (i.e., forward channels) by transmitting broadcast and unicast traffic in different downlink transmission time intervals. Accordingly, broadcast traffic may be transmitted in a first transmission time interval (TTI), while unicast traffic is transmitted in at least one TTI other than the first TTI. In general, the duration of each TTI is fixed. The number of OFDM symbols within a TTI may be different for broadcast traffic and unicast traffic. In general, a smaller number of OFDM symbols are carried in a broadcast TTI in order to allow for a longer cyclic prefix (CP).
In broadcast transmission over a cellular system, the same information content is transmitted simultaneously from all of the base stations of the cellular network or from a subset of all of the base stations. In the case of broadcast transmission from a subset of the base stations, the base stations transmitting the same content belong to a broadcast zone. Therefore, a receiver device (e.g., subscriber station, mobile station, mobile terminal, etc.) receiving the broadcast content potentially may receive signals from all or many of the base stations in the broadcast zone.
If all the base stations in the broadcast zone are synchronized and OFDM is used for transmission, a single frequency network (SFN) operation may be realized. In an SFN-based broadcast system, the signals from all the base stations in the broadcast zone may be collected at the receiver device without any interference apart from the background noise, plus any interference from cells not belonging to the broadcast zone. Therefore, signal-to-interference-plus-noise ratio (SINR) of the received broadcast signal can be improved. This allows for higher data rates and better recovery of the broadcast information.
Multiple Input Multiple Output (MIMO) schemes use multiple transmit antennas and multiple receive antennas to improve the capacity and reliability of a wireless communication channel. A MIMO system promises linear increase in capacity with the factor K, where K is the minimum of number of transmit (M) antennas and receive (N) antennas (i.e., K=min(M,N)). For example, four different data streams may be transmitted separately from the four transmit antennas. The transmitted signals are received at the four receive antennas. Some form of spatial signal processing is performed on the received signals to recover the four transmitted data streams. An example of spatial signal processing is V-BLAST, which uses successive interference cancellation to recover the transmitted data streams. Other variants of MIMO systems include schemes that perform some kind of space-time coding across the transmit antennas (e.g., D-BLAST) and also beam-forming techniques, such as spatial division multiple access (SDMA).
Good channel estimation is a key to the performance of any MIMO scheme. The MIMO channel estimation consists of estimating the channel gain and phase information for links from each of the transmit antennas to each of the receive antennas. The channel for an M×N MIMO system consists of a M×N matrix:
  H  =      [                                        a            11                                                a            12                                    …                                      a                          1              ⁢              N                                                                        a            21                                                a            22                                    …                                      a                          2              ⁢              N                                                            ⋮                          ⋮                          …                          ⋮                                                  a                          M              ⁢                                                          ⁢              1                                                            a                          M              ⁢                                                          ⁢              2                                                …                                      a            MN                                ]  where aij represents the channel gain from transmit antenna i to receive antenna j. In order to enable the estimations of the elements of the MIMO channel matrix, separate pilots may be transmitted from each of the transmit antennas.
U.S. patent application Ser. No. 11/414,814, filed May 1, 2006 and entitled “Apparatus And Method For Broadcast Superposition And Cancellation In A Multi-Carrier Wireless Network” discloses an apparatus and a method whereby broadcast and unicast traffic use the same subcarrier resources (or tones) in an OFDMA system. U.S. patent application Ser. No. 11/414,814 is assigned to the assignee of the present invention and is hereby incorporated by reference as if fully set forth herein. According to the teachings of U.S. patent application Ser. No. 11/414,814, the broadcast traffic is first decoded and cancelled before decoding of the unicast traffic takes place.
U.S. patent application Ser. No. 11/783,594, filed Apr. 23, 2007 and entitled “System And Method For Superposition Coding And Interference Cancellation In A MIMO System” discloses a system and method whereby MIMO broadcast transmissions are superimposed on MIMO unicast transmissions. U.S. patent application Ser. No. 11/783,594 is assigned to the assignee of the present invention and is hereby incorporated by reference as if fully set forth herein. According to the teachings of U.S. patent application Ser. No. 11/783,594, the MIMO broadcast traffic is first decoded and cancelled before decoding of the MIMO unicast traffic takes place. An effective cancellation operation requires reliable channel estimates and hence there is a need to improve the channel estimation performance in a system where broadcast and unicast traffics are superimposed.
However, in the conventional (or prior art) OFDM networks that incorporate MIMO transmission techniques, the broadcast pilot signals are transmitted at the same power as the broadcast traffic signals. As a result, the system experiences poor channel estimation performance and degraded system capacity.
Therefore, there is a need for improved OFDM (or OFDMA) transmission systems that make better use of the available downlink transmit power. In particular, there is a need for OFDM/OFDMA networks that use MIMO antenna systems to transmit superimposed broadcast and unicast data and that provide improved channel estimation performance and improved system capacity.